1. Field of the Invention
The present invention relates to a method of making tubular-steel reinforcements.
More particularly, the invention relates to a method of producing tubular-steel reinforcements for the interior of vehicle doors, especially the doors of automotive vehicles.
2. The Prior Art
The doors of automotive vehicles are essentially composed of an outer sheet-metal shell and an inner sheet-metal panel. In some cases the inner panel may be of synthetic plastic material or of another non-metallic material. In all instances, however, the outer shell and the inner panel bound between themselves a hollow space which is generally empty except for the window lift mechanism.
Since these doors do not offer much protection to persons in the vehicle, e.g., in the event of an accident, it is now widely customary--and in some countries required by law--to mount in the space between the outer shell and the inner panel a horizontally extending reinforcing member (also called an anti-intrusion member). If the inner panel is of metal, this reinforcing member may be welded to that side of the panel which faces the interior space of the door. In order not to contradict the efforts to make vehicles lighter and thus reduce fuel consumption, the reinforcing member is usually of tubular configuration rather than being a beam of solid cross-section. Of course, to offer adequate protection the reinforcing elements must meet certain minimum mechanical requirements which in some countries are regulated by law.
These minimum requirements are determined by the tensile strength .sigma..sub.B of the tubular reinforcing element, its limit of stretch .sigma..sub.S, its break elongation .delta..sub.5 and the deformation load. The deformation load is defined as the load expressed in kgm, which is required to produce a permanent bowing of the tubular reinforcing element (i.e., deflection of the originally straight element) of about 150 mm under conditions in which the ends of the element are loosely supported and the load is applied to the element in transverse direction at a point located substantially midway between the supports. The rule is that a tubular reinforcing element (installed in a vehicle door) can be expected to offer adequate protection against lateral impact on the door if its tensile strength .sigma..sub.B amounts to between 110-140 kg/mm.sup.2, its limit of stretch .sigma..sub.S to between 80-110 kg/mm.sup.2, its break elongation .delta..sub.5 is at least 13% and the deformation load is at least 240 kgm.
These acceptable values can be attained by making the tubular reinforcing elements from titanium-alloyed steel or from steel of the type St 52-3 as defined in DIN (German Industrial Standard) 17100. However, the use of these steel types requires a large number of process steps in connection with the preparation and shaping of the steel tubes. This is extremely time-consuming and expensive and is, therefore, highly undesirable from an economic point of view.